Scodaphoresis (or “SCODA”) is an approach that may be applied for concentrating and/or separating particles. SCODA may be applied, for example to DNA, RNA and other molecules. SCODA is described in:    PCT application WO 2005/072854A1 entitled Scodaphoresis and methods and apparatus for moving and concentrating particles;    PCT application WO 2006/081691A1 entitled Apparatus and methods for concentrating and separating particles such as molecules; and    Novel electrophoresis mechanism based on synchronous alternating drag perturbation, Marziali, A.; Pel, J.; Bizotto, D.; Whitehead, L. A. Electrophoresis 2005, 26, 82-89.    D. Broemeling, J. Pel, D. Gunn, L. Mai, J. Thompson, H. Poon, A. Marziali, An Instrument for Automated Purification of Nucleic Acids from Contaminated Forensic Sample JALA, 2008, 13, 40-48all of which are hereby incorporated herein by reference. SCODA can involve providing a time-varying driving field that applies forces to particles in some matrix and a time-varying mobility-altering field that affects the mobility of the particles in the matrix. The mobility-altering field is correlated with the driving field so as to provide a time-averaged net motion of the particles toward a focus area.
Some modes of SCODA apply electric fields to a medium in which particles respond to the electric fields in a non-linear manner. In such modes, suitably time-varying electric fields can cause certain types of particles to be focussed at locations within the medium. The following discussion of SCODA is intended as an illustrative example only. In many practical cases, the instantaneous velocity of a particle in a medium under the influence of an electric field is given by:{right arrow over (ν)}=μ{right arrow over (E)}  (1)where {right arrow over (ν)} is the velocity of the particle, {right arrow over (E)} is the electric field and μ is the mobility of the particle given, at least approximately, by:μ=μ0+kE  (2)where, μ0 and k are constants. Particles with larger values for k tend to be focussed more strongly than particles with smaller values for k.
In some cases, SCODA is performed by providing an electrical field having a rotating component and a quadrupole perturbation. The rotating component may be specified, for example, by:EX=E cos(ωt)  (3)andEy=E sin(ωt)  (4)where E is a magnitude of the electric field component that rotates at an angular frequency ω, and Ex and Ey are respectively x- and y-components of the rotating electrical field. The perturbing quadrupole component may be specified, for example, by:dEx=−dEqx cos(2ωt)  (5)anddEy=dEqy cos(2ωt)  (6)where dEx and dEy are respectively x- and y-components of the perturbing electrical field, x and y are distances from an origin at the center of the quadrupole field pattern and dEq is the intensity coefficient of the perturbing quadrupole field. With this SCODA field, the average radial velocity of a particle toward a focus location can be shown to be given by:
                                                        v              →                        r                    _                =                                            kEdE              q                        4                    ⁢                      r            →                                              (        7        )            where E is the magnitude of an electric field component that rotates at an angular frequency ω, Eq is a measure of the quadrupole component of the electric field that varies at an angular frequency of 2ω, and {right arrow over (r)} is a vector pointing toward the focus location and having a magnitude equal to the distance of the particle from the focus location.
The size of a spot into which particles can be focussed depends upon k as well as the rate at which the particles can diffuse in the medium as follows:
                              1          R                =                              k            D                                              (        8        )            where R is a radius of the focussed spot and D is a diffusion coefficient.
There is a need for practical and cost efficient methods and apparatus for introducing particles into a medium. There is a particular need for such methods and apparatus that are compatible with performing scodaphoresis to concentrate and/or separate the particles in the medium.